FIG. 10 illustrates the basic configuration of an ordinary color cathode ray tube used for a television receiver or the like. As shown in FIG. 10, the color cathode ray tube deflects an electron beam 3 emitted from an electron gun 2 with a deflection yoke 4 in the vertical direction and the horizontal direction, and reproduces an image by scanning the electron beam 3 over the entire screen.
A television receiver using such a color cathode ray tube is affected by the terrestrial magnetism. This means that on the earth, there is a magnetic field caused by the terrestrial magnetism, and when the terrestrial magnetism acts on the color cathode ray tube, then the electron beam 3 is distorted by the Lorentz force. As a result, mislanding occurs so that the electron beam 3 does not reach the predetermined location on the phosphor screen 9 provided on the inner side of the face panel 8, thus causing color displacements. To prevent these color displacements, ordinarily a degaussing coil is arranged around the color cathode ray tube, and a degaussing process removing the polarization of, for example, an inner magnetic shield 7 is carried out (see for example JP9 (1997)-135452A).
Moreover, JP6 (1994)-6817A proposes a technology in which a magnetic field correction coil is provided, and the degaussing process is carried out while intensifying the terrestrial magnetism.
However, in color cathode ray tubes having a striped phosphor screen, mislanding of the electron beam is caused by the Lorentz force Fx which acts on the electron beam in the horizontal direction (X-axis direction). This Lorentz force Fx affects the electron beam due to magnetic fields in the vertical direction (Y-axis direction) and in the tube axis direction (Z-axis direction), and can be expressed byFx=e(Byvz−Bzvy)  Equation 1wherein e is the charge of an electron, By is the magnetic flux density in the Y-axis direction, vy is the speed of the electron beam in the Y-axis direction, Bz is the magnetic flux density in the Z-axis direction (tube axis direction), and vz is the speed of the electron beam in the Z-axis direction (tube axis direction).
Here, the speed vy of the electron beam in the Y-axis direction and the speed vz of the electron beam in the Z-axis direction are determined by the operating voltage and the deflection angle of the electron beam. Consequently, to prevent color displacement, it is necessary to adjust the balance between the magnetic flux density By in the Y-axis direction and the magnetic flux density Bz in the Z-axis direction (tube axis direction) so as to reduce the Lorentz force Fx that the electron beam experiences in the horizontal direction (X-axis direction).
Owing to the structure of cathode ray tubes, the trajectory of the electron beam must be preserved, so that it is not possible to block the magnetic field in the Z-axis direction (tube axis direction) with an inner magnetic shield. Thus, when the terrestrial magnetism acts in the tube axis direction (Z-axis direction), most of the magnetism cannot be blocked, so that the magnetic flux density Bz in the Z-axis direction (tube axis direction) becomes large. In this case, in order to reduce the Lorentz force Fx experienced by the electron beam in the horizontal direction (X-axis direction), it is necessary to produce a By that is large enough to cancel the Bz term (Bzvy), but with the conventional technology of the above-mentioned JP9 (1997)-135452A, it is not possible to produce a By that is sufficiently large to cancel the Bz term (Bzvy).
Furthermore, in the technology disclosed in JP H06-6817A, a magnetic field correction coil is provided so as to perform a degaussing process while intensifying the terrestrial magnetism, but at least a pair of magnetic field correction coils are necessary to intensify the terrestrial magnetism. Consequently, using the technology disclosed in this publication, the number of components increases, and there is the problem that costs rise. Furthermore, if that technology is used, there is also the problem that it is not possible to reduce the influence of the terrestrial magnetism uniformly across the entire screen, since the correction response differs between the screen corner portions and other regions.
It is thus an object of the present invention to present a color cathode ray tube provided with a degaussing function that can reduce at low cost the influence of magnetic fields remaining after an ordinary degaussing process and of external magnetic fields on the trajectory of an electron beam. More specifically, it is an object of the present invention to present a color cathode ray tube that has a striped phosphor screen and that is provided with a degaussing function with which color displacements caused by the tube axis (Z-axis) component of the terrestrial magnetism can be decreased at low cost.